With recent development of portable devices such as a notebook personal computer and a mobile phone, there is an increasing demand for batteries used as a power source for those devices. The batteries used for these devices are required to have high energy density and excellent cycle performance. To meet these demands, new materials for the active materials of positive electrodes and negative electrodes have been developed. Particularly, for achieving high capacity non-aqueous electrolyte secondary batteries, materials containing Si element (Si-based materials) and materials containing Sn element (Sn-based materials) have been gaining attention as an electrode active material. For example, the theoretical discharge capacity of a Si simple substance is about 4199 mAh/g, which is about 11 times the theoretical discharge capacity of graphite.
However, the Si-based materials and the Sn-based materials undergo drastic structural change and expand when absorbing lithium ions. As a result, the active material particles crack, and the active material is separated from the current collector. This reduces electron conductivity between the active material and the current collector, thereby degrading battery performance such as cycle performance.
Therefore, compounds containing Si or Sn (for example, oxides, nitrides, and oxynitrides) have been proposed for use as the active material. The discharge capacities of these active materials are slightly lower than the discharge capacity of the simple substances, but expansion and contraction of the active material are decreased.
Easing the expansion stress at the time of lithium ion absorption by providing spaces between the active material particles in advance has also been proposed. For example, in Patent Document 1, forming an active material layer of columnar particles on a current collector in a predetermined pattern by, for example, a photoresist method and a plating method has been proposed. By forming the columnar active material, gaps are formed in the active material layer, which eases the expansion stress of the active material.
In Patent Document 2, forming an active material layer on a current collector with the surface roughness Ra of 0.01 μm or more to increase the contact area between the active material and the current collector has been proposed. By increasing the contact area between the active material and the current collector, the active material separation from the current collector can be inhibited.
Furthermore, negative electrode deformation is also a serious problem when using, for example, a Si-based material or a Sn-based material as the negative electrode active material. When such a negative electrode active material absorbs and desorbs lithium ions at the time of charging and discharging, a large stress is caused by the expansion and contraction of the negative electrode active material. Thus, the negative electrode is distorted, causing wrinkling on the current collector, and cutting of the current collector. This creates space between the negative electrode and the separator, which causes varied distance between the positive electrode and the negative electrode, leading to nonuniform charge and discharge reaction. As a result, a local property decline is caused in the battery.
To solve the above-described problem, for example, providing gaps in the active material layer has been proposed. For example, in Patent Document 3, forming gaps between the active material particles by depositing the active material particles on a current collector having an uneven surface has been proposed. In Patent Document 4, a negative electrode with secondary particles having gaps therebetween, formed by depositing the active material particles on the current collector such that primary particles aggregate to form the secondary particles has been proposed.
As described above, by providing gaps in the active material layer, the stress resulting from the active material expansion and contraction is eased, inhibiting distortion of the negative electrode. Thus, the current collector can be prevented from wrinkling, and the negative electrode active material can be prevented from being separated from the current collector; therefore, decline in battery capacity and cycle performance can be inhibited.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-127561    Patent Document 2: WO 01/31722    Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-313319    Patent Document 4: Japanese Laid-Open Patent Publication No. 2006-155958